With regard to the reprocessing of irradiated nuclear fuel, the Purex process is usually applied. Further developments of the Purex process lead to a situation wherein organic extraction agents and aqueous phases are increasingly circulated. This intensive recycling of the process media does, however, necessitate effective regeneration steps for both phases. The foregoing is made more difficult by the fact that the purification methods used should result in the least amount of waste possible.
For cleaning the aqueous phase, a kerosene wash is used. The process flows occurring in the Purex process always contain some organic extraction medium in dissolved and finely suspended form. Upon further processing of the aqueous process solutions, the organic extraction medium residues, for example tributyl phosphate (TBP), would become concentrated and cause problems.
During the kerosene wash mentioned above, the aqueous phase comes in contact with an organic thinner (kerosene). The entrained residues of organic extraction medium, by reason of their better solubility in the organic phase, are washed out of the aqueous solution. The extraction medium residues are thus effectively isolated.
From the text "Chemie der nuklearen Entsorgung", Part III, Verlag Karl Thiemig, Munich, Volume 91, page 146, it is known to use mixer-settlers for the kerosene washing process. It has been ascertained that installations with single-stage mixer-settlers ensure only an inadequate TBP separation.
Multi-stage mixer-settlers are box-shaped containers which comprise a plurality of closely adjacent mixing and settling chambers. The aqueous phase and the organic phase are blended in the mixing chamber by agitation. The mixture flows into the settling chamber in which organic and aqueous phases separate following mass transfer. The organic phase flows into an adjacent mixing chamber while the aqueous phase flows into the oppositely adjacent mixing chamber of the next stage. Because of a plurality of serially connected mixer-settler stages, this process is repeated until such time as the desired separating effect has been achieved. The mixer-settler banks have a pumping-mixing agitator in the mixing chamber.
The sluggish way the mixer-settlers operate brings with it the disadvantage of long response times in the case of stationary operating situations. The state of hydrostatic equilibrium in which all chambers are in relation to one another can be disturbed, for example, by changes in concentration or density. Then one of the two phases is displaced from the chambers and the countercurrent is stopped. The known mixer-settlers do not permit a free choice of the continuous phase either, which would be desirable for reasons of mass transfer direction, mass transfer area, phase separation performance, et cetera.
A Pu-bearing phase also occurs as an aqueous process flow which is to be purified. This Pu-containing aqueous process flow ought to be cleansed with kerosene to remove extraction medium residues of tributyl phosphate. Where the known box-like mixer-settlers are concerned, only a small overall height is admissible on grounds of criticality, so that throughput is quite limited. In addition, a clearly defined working pattern is impossible with respect to recycling of the washing phase quantity. The flow of product is therefore only incompletely cleansed of extraction medium residues. Furthermore, with the low overall height which is possible, fluctuations in flow or dispersion build-up as well as deposits of mull in the settling chambers quite frequently lead to hydraulic disturbances.
In order to achieve minimal secondary waste and optimum extraction, an internal and defined recycling of the washing phase is sought. In this way, the supply of external washing phase can be minimized because the washing phase is used more effectively.
Multi-stage mixer-settlers with tubular settling chambers guarantee the required throughput geometrically-critically and do not have to be heterogeneously contaminated by incorporated structures. With a tube diameter of 150 mm, Pu-concentrations of up to 350 g Pu per liter can be handled. If, then, a controlled internal recycling of the washing phase is also achieved, then extraction can be optimally performed. The washing flow, for example kerosene, can be reduced generating a considerably smaller flow of waste.